This invention relates to systems and methods for controlling cable suspended, payload transfer systems. More particularly, this invention relates to anti-sway control systems and methods for a payload undergoing both horizontal trolley and vertical hoisting motions.
Gantry-style cranes are used extensively for the transfer of containers in port operation. Typically, a crane has two inputs in the form of velocity commands. These two velocity commands independently control horizontal trolley and vertical hoisting motions of a payload. Undesirable swaying of a payload at the end of the transfer is one difficulty in accomplishing a transfer movement. Loading or unloading operations cannot be accomplished when a payload is swaying. Presently, only an experienced operator can efficiently bring the container to a swing-free stop. Other operators must wait for the sway to stop. Typically, the time spent waiting for the sway to stop, or the various maneuvers to fine position the load, can take up to one-third of the total transfer time.
Various prior art patents teach sway reduction systems. These patents relate to different aspects of payload transfer with reduced sway. For example, several patents describe operation in autonomous mode where system uses the starting and ending positions of the payload to generate the necessary control signals to achieve the payload transfer. Other non-autonomous systems attempt to minimize the amount of payload sway while following the operator""s commands for horizontal trolley and vertical hoisting motions.
Autonomous systems are suitable for structured environments where positions of a payload are well identified, In a typical port environment, a container""s position depends on the relative positioning of the ship relative to the crane. Therefore, the position of the container is rarely precisely known. In such an environment, a non-autonomous mode of operation is preferred. The present invention relates to such non-autonomous systems.
Several references disclose non-autonomous modes of operation. Many of these references use a fixed-length pendulum model as the basis for their sway reduction method and/or system. Consequently, these strategies do not eliminate sway when the cable length changes during horizontal motion. Several other references handle the effect of changing vertical cable length by using approximations. The present invention uses the full dynamical equation of a crane system without approximation in order to avoid error and to eliminate sway. In particular, the present invention uses cancellation acceleration for sway control. The computation of a cancellation signal is exact as it is based on the full dynamical equation of the crane model. This is particularly significant during simultaneous trolley and hoist motions. For the ease of discussion, the angle of sway of the load and the velocity of sway of the load are shown as xcex8 and {dot over (xcex8)}, respectively, and the acceleration of the trolley is referred to as {umlaut over ("khgr")}. All control systems use the horizontal acceleration of the trolley as the control for sway. Hence, horizontal acceleration is also termed the control.
There are two general approaches for sway minimization. In first approach, the trolley acceleration is given in the form {umlaut over (x)}=r+k1xcex8+k2{dot over (xcex8)} or something similar. Here, the value r is a time function that depends on the desired motion of the trolley. The use of this approach introduces additional damping into the system to control sway. The resultant system can be made to have any desirable damping ratio and natural frequency using the appropriate values of k1 and k2.
Several references adopt this first approach. These references differ in the profile of the motion dependent time function, r, and the specific procedure by which values of the damping ratios, k1 and k2, are determined. In the U.S. Pat. No. 5,443,566 to Rushmer, sway angle and sway angle velocity are estimated using a fixed-length cable model of the crane. Estimates of the sway angle, xcex8, and the sway angle velocity, {dot over (xcex8)}, are used together with the input velocity demand from the operator, {dot over (x)}d, to compute the control signal {umlaut over (x)}=k1({dot over (x)}dxe2x88x92{dot over (x)})+k2xcex8+k3{dot over (xcex8)}. In U.S. Pat. No. 5,490,601 to Heissat et al., the control signal is {umlaut over (x)}=k1xcex8+k2{dot over (xcex8)}+k3(xdxe2x88x92x). Sets of k1, k2, and k3 are determined experimentally at various lengths of the cable. The exact values of k1, k2, and k3 for a particular cable length are interpolated from these experimental sets using gain scheduling, or some form of fuzzy or neural network control. In U.S. Pat. No. 5,878,896 to Eudier et al., the speed demand send to the trolley is of the form {dot over (x)}d=k1xcex8+k2{dot over (xcex8)}+k3(xdxe2x88x92x) where xd is the desired position of the trolley. The values of k1, k2, and k3 are determined experimentally.
This first approach can effectively damp out sway. The approach is based on standard mechanism of feedback and is therefore robust against model inaccuracies. The main disadvantage of this approach is its lack of intuitive control by the operator. As the trolley acceleration depends on xcex8, {dot over (xcex8)} and the operator""s desired velocity, the motion of the trolley can be unpredictable and counter-intuitive to the operator. As a result, several manuevers may be needed to bring the system to a proper stop. As such, this first approach is suitable for an unmanned crane in a structured environment where payload position is well identified.
A second approach is based on the principle of sway cancellation. This is the mechanism used by most human operators for sway damping. The basic idea of this approach for a fixed-length pendulum is described in Feedback Control Systems, McGraw-Hill, New York, 1958, by O. J. Smith. In a fixed-length pendulum, the sway motion is a nearly sinusoidal time function with a frequency xcfx89, defined by xcfx89={square root over (g/l)}. Suppose that a short pulse of horizontal acceleration is applied at time t=0, this pulse will induce a sway oscillation of frequency xcfx89. It is possible to cancel this oscillation using a second short pulse of the same magnitude and duration applied at time t=xcfx80/xcfx89. After the application of the second pulse, the system will have no sway for the time thereafter. This method, known as double-pulse control or cancellation control, gives the shortest possible settling time for a constant length cable. While this method is readily applicable to a fixed-length pendulum, extensions to pendulums with varying cable length extension are not easy.
Several references teach the general approach of cancellation control. In U.S. Pat. No. 4,756,432 to Kawashima et al., it appears that double-pulse control is used in both the acceleration and deceleration phases of the trolley motion. For a specified final trolley location, the timing and magnitude of these pulses are computed based on a fixed-length pendulum. One double-pulse is used in each of the acceleration and deceleration phases. In between these two phases, the trolley travels at constant velocity and does not sway. In order for this method to work, the operator must provide the final position of the trolley to accurately determine the timing and magnitude of the pulses. This system works reasonably well when the cable length is constant during horizontal motion.
In U.S. Pat. No. 5,219,420 to Kiiski et al., it appears that the sway angle is measured and a best fit sinusoidal time function is made of the sway motion. With this estimated sinusoidal function, a cancellation pulse is generated to eliminate sway. The method assumes the presence of only one sinusoidal frequency. As such, the method is not effective for systems which the cable length changes during horizontal motion of the trolley.
In U.S. Pat. No. 5,960,969 to Habisohn, a digital filter is used for damping oscillation. It appears that components of the input signal close to the crane oscillation frequency are filtered off. In particular, the filtered output is a simple average of the input signal and the input signal delayed by a one-half period of the load pendulum motion. Several other filter versions based on linear combinations of input signals with different delays are used. These input signals are computed using the constant length version of the crane equation.
The methods in the above references rely on constant-length pendulum systems for cancellation. The following references review other attempts to extend cancellation control to varying-length cable systems.
In U.S. Pat. No. 5,785,191 to Feddema et al., an impulse response filter and a proportional-integral controller is disclosed for the control of the crane under the operator""s input. The impulse filter based on a digital implementation of an inverse dynamics idea is commonly used in the study of control systems. In this case, a feed forward controller is used to cancel the dynamics of the crane system and to introduce user-defined dynamics.
In U.S. Pat. No. 5,127,533 to Virrkkumen, an attempt to adapt a control design for a crane having a fixed-length cable to a control design for a crane having a variable-length cable is disclosed. It is well known that the period of oscillation of a pendulum is proportional to the square root of the pendulum length. The reference shows that a control signal applicable for a crane having a fixed cable length, referred to as L1, can be used for the crane having another cable length, referred to as L2, by a suitable delay. For example, suppose the control signal is based on a crane design for a fixed length, L1, and the control signal is applied at a first time, t1. Virrkkumen teaches that the same effect can be achieved on the crane having another fixed length, L2, when the control signal is applied at time:       t    2    =            t      1        *                            L          2                                      L          1                    
While the method of Virrkkumen is reasonable for two fixed-length pendulums, it is not accurate for a single pendulum, or a single crane, undergoing a change in cable length. For example, the hoisting rate of the cable affects the sway angle, and this is not accounted for in Virrkkumen. In addition, there is the uncertainty in the determination of the second cable length, L2, as the length may be changed continuously during a typical horizontal motion.
In U.S. Pat. No. 5,526,946 to Overton, the basic sway control teaching is an extension of Kawashima and Virrkkumen. Instead of a fixed double-pulse at the acceleration and deceleration phases, Overton teaches the use of double-pulse whenever there is a change in the velocity input. For a sequence of continuously changing velocity input, two sequences of pulses are generated. The first sequence is synchronized with the input velocity change. The second sequence is also generated and then stored. The second sequence corresponds to a second pulse of the double pulse control method. Each of the signals in the second sequence is applied to the horizontal acceleration of the trolley at about one-half of a pendulum period after the signal in the first sequence. Overton adapts Virrkkumen in calculating the timing of these signals. This second sequence is processed (or sent as trolley acceleration) at a variable rate proportional to the current length of the cable. The shorter the cable length, the faster the entries of the sequence are sent out. As Overton is an adaptation of Virrkkumen, it suffers from similar deficiencies.
The present invention uses double pulse control for sway cancellation. However, the present invention differs from the references above in several significant aspects. The present invention computes the exact timing and magnitude of a second pulse using the full dynamic equation of the crane system. The application of this second pulse eliminates sway even during changing cable length. This precise cancellation pulse computation is crucial for proper sway elimination. The present invention also ensures that physical constraints, in the form of acceleration and velocity limits of the trolley, are never exceeded. The present invention also includes a feedback mechanism to eliminate sway due to external forces, such as wind load and other external disturbances.
An object of the present invention is to provide a computer-controlled system for the control of sway in a crane. The present invention uses cancellation pulses for sway control. Sway is incrementally canceled after being induced by prior commands for trolley acceleration. The timing and magnitude of these cancellation pulses are critical components to the effectiveness of the present anti-sway method. The present invention also takes into account the full dynamic effect of the varying cable length in the computation of these cancellation signals.
Another object of the present invention is to determine precise cancellation acceleration pulses. By using a family of ordinary differential equations, the precise cancellation acceleration pulses are determined.
A further object of the present invention is the operation of the anti-sway system and method within the acceleration and velocity limits of the trolley drive system. Sway control can be adversely affected when acceleration saturation or velocity saturation of the trolley drive system occurs. The present invention includes a system and method to ensure the proper functioning of the anti-sway mechanism within these limits.
Yet another object of the present invention is to provide an anti-sway controller unit or kit for incorporation into an existing crane system. The anti-sway controller unit is connected between the operator""s velocity commands and the existing variable speed controllers. This anti-sway controller follows an operator""s input commands for both horizontal trolley travel and vertical payload hoisting. The controller unit can be switched off, if so desired, to restore manual operator control of the crane.
Still another object of the present invention is residual sway elimination. Using sensory measurement of the sway the present invention is further enhanced by a feedback mechanism. This feedback mechanism complements the anti-sway controller and eliminates residual sway due to external factors.
Still other objects of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein a preferred embodiment of the invention is shown and described by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.